sylph
Explanation
This comic is a fake historical linguistics chart showing "English words that rhyme with 'sylph'" over the entire history of the English language, from 500 AD to the present.
The chart shows that during Old English (Englisc), there was exactly one word that rhymed with "sylph": the word "ilf," which was used as an alternate form of "elf." That rhyming word then disappeared during Middle English and Early Modern English, leaving zero rhymes for centuries -- a period the comic dramatically labels "the long age of darkness."
Then, in "Post-2008 English," the count jumps back to one, because the word "MILF" was added to the Oxford English Dictionary.
The humor operates on multiple levels. First, there is the absurdity of presenting what is essentially a vulgar slang observation in the form of a serious, scholarly historical linguistics chart, complete with proper periodization of English language eras. The visual format implies rigorous academic research, but the punchline is a crude acronym from internet culture.
Second, the comic plays on the genuine oddity that "sylph" (a mythological air spirit, coined by Paracelsus) is an extremely difficult word to rhyme -- it really does have almost no perfect rhymes in English. The fact that the only modern solution is a piece of slang that stands for "Mom I'd Like to F***" makes the scholarly framing even funnier.
The caption "The long age of darkness is over" treats the addition of "MILF" to the dictionary as a triumphant civilizational achievement, as though humanity had been suffering for centuries under the burden of having no rhyme for "sylph." This is a parody of how we sometimes frame minor cultural developments in grandiose historical terms.